I did some work as part of my final year undergraduate project and wrote a paper which has now been accepted at a conference. Though I am now working in the industry, I have listed my affiliation as my University because that is where I did my work as a student. So, for registering in the conference, do I have to pay the full registration fee (because I am currently not a student) or can I pay the student fee (because I did the work during my undergraduate days)?
You're not a student now
The fees are based on the status of the participants, not the status of the papers. Often, organizers will require some evidence of student status (such as a current student ID) in order to be eligible for the reduced price.
It may be reasonable to get the student fee if you're a student at the time of registration but graduate before the actual conference - as the process often takes multiple months, but if it's something earlier, then you're out of luck.
Conferences usually grant discounts to financially disadvantaged participants, such as students or attendees from developing countries, because they want to enable as many people as reasonably possible to attend the conference. From another point of view, discounts are a social redistribution of wealth, with the rich attendees supporting the poorer ones. In theory, the organisers could negotiate each of this discounts individually, as there usally are no constraints regarding this imposed by some higher authority¹. Of course, this would cost more time (and thus money) than it saves and thus you usually have discounts for certain groups, such as students.
So, it’s up to the organisers whether they regard you as a student or not and to be really sure, you have to ask them. Given that falsely registering as a student could be considered fraud, you certainly should clarify the situation before doing this. However, you might want to take the reason behind those fees into account and just pay the undiscounted fee, if you are doing well financially, which also saves the organisers some time. On the other hand, if you aren’t, this might be worth mentioning when applying for a student discount, though you technically aren’t a student anymore.
¹ apart from anti-discrimination laws and similar